The disk you inserted was not readable by this computer error

The error message "The disk you inserted was not readable by this computer" is a common issue faced by Mac users when trying to connect external drives, USB flash drives, SD cards, or other storage devices to their computers. This error can be caused by several reasons, including corrupted file systems, unsupported formats, hardware failures, or improper ejection from another device. When this message appears, it means the Mac cannot recognize the disk in its current state and is unable to access the data stored on it.

One of the first things you should do when encountering this error is not to click "Initialize" right away. Clicking that option will open Disk Utility, but initializing or formatting the drive will erase all existing data. If you have important data on the drive, you should consider data recovery methods before proceeding with any formatting or repair operations.

A simple but often effective first step is to remove the disk and reconnect it. Sometimes, the error may be triggered due to a loose connection or a temporary glitch. Try using a different USB port or a different cable, especially if you're using an external drive with its own cable. If the error persists, try connecting the disk to another Mac or even a Windows computer to see if it’s recognized. This will help determine if the issue is with the disk itself or your Mac’s hardware or software.

If the disk is not recognized on any computer, the file system may be corrupted or the device may have physical damage. However, if it works on another system, the issue could be limited to your Mac’s configuration or compatibility with the file system. For example, a drive formatted in NTFS might not be fully supported on macOS without third-party drivers. Similarly, drives formatted with unsupported or proprietary formats from digital cameras or other devices may not be recognized properly.

Disk Utility is a built-in tool on macOS that can help diagnose and fix minor disk issues. Open Disk Utility from the Applications > Utilities folder. Check if the problematic disk is listed in the sidebar. If it is, select it and click “First Aid” to run a check and attempt repairs. This process won’t erase your data and can fix minor errors in the disk’s structure. If First Aid is successful, your drive might be accessible again.

If Disk Utility can see the drive but First Aid fails to repair it, you may need to use a third-party data recovery software. These tools can scan the corrupted disk for recoverable files and allow you to save them elsewhere. Only after recovering important files should you proceed to reformat the drive.

In cases where the disk doesn’t show up in Disk Utility at all, the problem might be hardware-related. The drive itself could be failing, or the issue could be with the USB port or cable. Try using another external enclosure or adapter if the drive is a SATA or NVMe SSD or HDD. If it’s a removable card like an SD or microSD, test it with a different card reader.

After data recovery, if you're ready to use the disk again, you can initialize it. From Disk Utility, select the disk and click “Erase.” Choose an appropriate format like APFS or Mac OS Extended (Journaled) for macOS use, or exFAT if you need cross-platform compatibility. Give the disk a name and complete the process to make the drive usable again.

To prevent this issue in the future, always eject external drives safely before unplugging them. Avoid disconnecting storage devices while files are transferring, and regularly back up your important data to avoid loss in case of corruption or hardware failure. Keeping your macOS updated and using quality hardware also reduces the risk of encountering unreadable disk errors